The present invention relates, in general, to offshore platforms fabricated from tubular members, and more particularly to the problem of reducing or eliminating the effect of hydrostatic pressure on the design of tubular members immersed in sea water as well as a method of deballasting such tubular members.
As is well known to those practiced in the art, the structural members of an offshore platform which are immersed in sea water usually are tubulars and must be designed to resist net external hydrostatic pressure in combination with whatever other loads are imposed on the structural members. The structures normally must be designed to float so that they can be installed using controlled ballasting techniques. Thus, during installation most of the structural members have one atmosphere of pressure on the inside and are exposed to the ambient pressure on the outside. After installation, the structural members are normally left void because it would be undesirable and impractical to flood the structural members after the installation. Thus, most of the structural members that are immersed in sea water are exposed to the full ambient hydrostatic pressure during installation and during the life of the structure. For convenience and brevity, the terms "tubular", "tubulars", "tubular members", and "tubular structures" will be used throughout this description to generally refer to the legs and truss members used in offshore structures.
The hydrostatic pressure induces hoop compression in the tubulars. Also, the hydrostatic end force induces axial compression in the tubulars. The stresses in the structural members caused by the hydrostatic pressure require extra wall thickness and often require that compression reinforcing rings be attached to the structural members at intervals along their lengths. These rings prevent instability and subsequent flattening of the tubular under the action of hydrostatic pressure. The deeper the water and the larger the member diameter, the greater is the expense related to hydrostatics. Ultimately, in deep enough water it becomes impractical to use void members, which means that extra buoyancy must be provided higher up in the structure to float the structure. In short, the design of the structure for hydrostatics becomes increasingly expensive with increasing depth and finally becomes prohibitive.
Another approach to the hydrostatic problem has been considered, but it has been rejected because of concerns over safety. If the members that are most affected by hydrostatics were filled with compressed air in the fabrication yard, then after installation the internal and external pressures acting on the tubulars could be balanced, thereby eliminating the hydrostatic problem. However, to satisfy concerns for safety these tubulars would have to be designed and rated as pressure vessels, which makes the idea too costly to use.
It is also known to utilize installation plans which require the deliberate flooding of certain tubular structures, followed by deballasting or blowing out of the flooded tubulars. Usually one or two chambers in each tubular leg of a structure are flooded although any member might be selected depending on the properties of the structure made up of the tubular members. As shown in FIG. 2, deballasting is done by blowing water, shown at 32, out with compressed air or nitrogen 100, supplied by a dedicated boat through a piping system 110 built for the purpose. The jacket leg 112 is shown to have upper and lower closures 114 and 116, and water 32 is discharged through a lower valve 118 into the ocean. The amount of pressure to be applied to the air or nitrogen 100 depends on how far the tubular 112 extends below the sea level, shown at 120.